Rich Non-centrosymmetry in a Na-U-Te Oxo-System Achieved under Extreme Conditions

Understanding of the structural chemistry in the uranium oxo-tellurium system under HT/HP conditions

The study of phase formation in the U-Te-O systems with mono and divalent cations under high-temperature high-pressure (HT/HP) conditions has resulted in four new inorganic compounds: K2 [(UO2) (Te2O7)], Mg [(UO2) (TeO3)2], Sr [(UO2) (TeO3)2] and Sr [(UO2) (TeO5)]. Tellurium occurs as TeIV, TeV, and TeVI in these phases which demonstrate the high chemical flexibility of the system. Uranium VI) adopts a variety of coordinations, namely, UO6 in K2 [(UO2) (Te2O7), UO7 in Mg [(UO2) (TeO3)2] and Sr [(UO2) (TeO3)2], and UO8 in Sr [(UO2) (TeO5)]. The structure of K2 [(UO2) (Te2O7)] is featured with one dimensional (1D) [Te2O7]4- chains along the c-axis. The Te2O7 chains are further linked by UO6 polyhedra, forming the 3D [(UO2) (Te2O7)]2- anionic frameworks. In Mg [(UO2) (TeO3)2], TeO4 disphenoids share common corners with each other resulting in infinite 1D chains of [(TeO3)2]4- propagating along the a-axis. These chains link the uranyl bipyramids by edge sharing along two edges of the disphenoids, resulting in the 2D layered structure of [(UO2) (Te2O6)]2-. The structure of Sr [(UO2) (TeO3)2] is based on 1D chains of [(UO2) (TeO3)2]∞2− propagating into the c-axis. These chains are formed by edge-sharing uranyl bipyramids which are additionally fused together by two TeO4 disphenoids, which also share two edges. The 3D framework structure of Sr [(UO2) (TeO5)] is composed of 1D [TeO5]4− chains sharing edges with UO7 bipyramids. Three tunnels based on 6-Membered rings (MRs) are propagating along [001] [010] and [100] directions. The HT/HP synthetic conditions for the preparation of single crystalline samples and their structural aspects are discussed in this work.

Advances and perspectives of actinide chemistry from ex situ high pressure and high temperature chemical studies

High pressure high temperature (HP/HT) studies of actinide compounds allow the chemistry and bonding of among the most exotic elements in the periodic table to be examined under the conditions often only found in the severest environments of nature. Peering into this realm of physical extremity, chemists have extracted detailed knowledge of the fundamental chemistry of actinide elements and how they contribute to bonding, structure formation and intricate properties in compounds under such conditions. The last decade has resulted in some of the most significant contributions to actinide chemical science and this holds true for ex situ chemical studies of actinides resulting from HP/HT conditions of over 1 GPa and elevated temperature. Often conducted in tandem with ab initio calculations, HP/HT studies of actinides have further helped guide and develop theoretical modelling approaches and uncovered associated difficulties. Accordingly, this perspective article is devoted to reviewing the latest advancements made in actinide HP/HT ex situ chemical studies over the last decade, the state-of-the-art, challenges and discussing potential future directions of the science. The discussion is given with emphasis on thorium and uranium compounds due to the prevalence of their investigation but also highlights some of the latest advancements in high pressure chemical studies of transuranium compounds. The perspective also describes technical aspects involved in HP/HT investigation of actinide compounds.

Achieving and Stabilizing Uranyl Bending via Physical Pressure

Applying physical pressure in the uranyl-sulfate system has resulted in the formation of the first purely inorganic uranyl oxo-salt phase with a considerable uranyl bend: Na₄[(UO₂)(SO₄)₃]. In addition to a strong bend of the typically almost linear O═U═O, the typically equatorial plane is broken up by two out-of-plane oxygen positions. Computational investigations show the origin of the bending to lie in the applied physical pressure and not in the electronic influence or steric hindrance. The increase in pressure onto the system has been shown to increase uranyl bending. Furthermore, the phase formation is compared with a reference phase of a similar structure without uranyl bending, and a transition pressure of 2.5 GPa is predicted, which is well in agreement with the experimental results.

Crystal growth of novel 3D skeleton uranyl germanium complexes: influence of synthetic conditions on crystal structures

Five centrosymmetric uranyl germanate compounds, K₈BrF(UO₂)₃(Ge₂O₇)₂, Rb₆(UO₂)₃(Ge₂O₇)₂·0.5H₂O, Cs₆(UO₂)₂Ge₈O₂₁ and A⁺₂(UO₂)₃(GeO₄)₂ (A⁺ = Rb⁺, Cs⁺), were synthesized in this work. K₈BrF(UO₂)₃(Ge₂O₇)₂ and Rb₆(UO₂)₃(Ge₂O₇)₂·0.5H₂O were obtained under mixed KF-KBr flux and hydrothermal conditions, respectively. Both structures crystallized in the triclinic P1[combining macron] space group and have similar anionic frameworks featuring novel hexagon shaped 12-membered channels. The condensation of two different types of SBU [UGe₄] pentamers (A) and (A2) results in the formation of K₈BrF(UO₂)₃(Ge₂O₇)₂ and Rb₆(UO₂)₃(Ge₂O₇)₂·0.5H₂O frameworks. Cs₆(UO₂)₂Ge₈O₂₁ was obtained from a CsF-CsCl high temperature flux, and it also crystallized in the centrosymmetric triclinic P1[combining macron] space group. The structure of Cs₆(UO₂)₂Ge₈O₂₁ has a novel oxo-germanate layer composed of germanate tetrahedra and trigonal bipyramids. Two new SBU types, (4²·5²-A2) and (5⁴-A2) [UGe₄] pentamers, were found in the structure of Cs₆(UO₂)₂Ge₈O₂₁. A⁺₂(UO₂)₃(GeO₄)₂ (A⁺ = Rb⁺, Cs⁺) were synthesized by a high temperature/high pressure (HT/HP) technique, and both structures with oval-shaped 12-membered channels crystallized in the centrosymmetric orthorhombic Pnma space group. The extreme conditions led to the formation of [U₂Ge₂] tetramers (E), which consist of 7-coordinated U and 5-coordinated Ge. Different synthetic methods of uranyl germanate compounds resulted in a distinct coordination environment of the uranyl cations and a variety of U[double bond, length as m-dash]O and U-O bond lengths, further affecting the dimensionality and types of uranyl units and SBUs. The Raman and IR spectra of the five new phases were collected and analyzed.

Structural and Spectroscopic Investigation of Novel 2D and 3D Uranium Oxo-Silicates/Germanates and Some Statistical Aspects of Uranyl Coordination in Oxo-Salts

Synthesis, structural and spectroscopic characterization, and topological analysis of five novel uranyl-based silicates and germanates have been performed. The open-framework K₄(UO₂)₂Si₈O₂₀·4H₂O has been synthesized under hydrothermal conditions and is based upon [USi₆] heptamers interconnected via edge-sharing. Its structure is composed of sechser silicate layers with 4-, 8-, and 16-membered rings. The largest 16-membered rings have an average dimension of ∼8.93 × 9.42 Ų. β-K₂(UO₂)Si₄O₁₀ has been obtained by the high-temperature flux growth method. Its 3D framework contains a loop-branched sechser single layer with 4- and 8-membered rings and consists of the same [USi₆] heptamers as observed in K₄(UO₂)₂Si₈O₂₀·4H₂O. Na₆(UO₂)₃(Si₂O₇)₂ has also been synthesized from melted fluxes and represents a 2D layer structure composed by [USi₄] pentamers. Two iso-structural compounds A⁺(UO₂)(HGeO₄)·H₂O (A⁺ = Rb⁺, Cs⁺) were synthesized via the hydrothermal method, and their structures are of the α-uranophane type. The 2D layers consist of [U₂Ge₂] tetramer secondary building units (SBUs). The Raman spectra of all novel phases were collected, and bands were assigned according to the existing oxo-silicate rings and oxo-germanium units. Additionally, we performed a statistical investigation of the local coordination of uranyl ions in all known inorganic structures with different oxo-anions (TO_x, T = B³⁺, Si/Ge⁴⁺, P/As⁵⁺, S/Se/Te⁶⁺, Cr/Mo/W⁶⁺, P/As³⁺, and Se/Te⁴⁺). We found a direct correlation between the ionic potential of the central cations T in oxo-anions in their higher oxidation states and the coordination number of uranyl groups.

Investigation of reactivity and structure formation in a K-Te-U oxo-system under high-temperature/high-pressure conditions

The high-temperature/high-pressure treatment of the K-Te-U oxo-family at 1100 °C and 3.5 GPa results in the crystallization of a series of novel uranyl tellurium compounds, K₂[(UO₂)₃(Te^IVO₃)₄], K₂[(UO₂)TeO₁₄], α-K₂[(UO₂)Te^VIO₅] and β-K₂[(UO₂)Te^VIO₅]. In contrast to most of the reported uranyl compounds which are favorable in layered structures, we found that under extreme conditions, the potassium uranyl oxo-tellurium compounds preferably crystallized in three-dimensional (3D) framework structures with complex topologies. Anion topology analysis indicates that the 3D uranyl tellurite anionic framework observed in K₂[(UO₂)₃(Te^IVO₃)₄] is attributable to the additional linkages of TeO₃ polyhedra connecting with TeO₄ disphenoids from the neighboring U-Te layers. The structure of K₂[(UO₂)TeO₁₄] can be described based on [UTe₆O₂₆]^22- clusters, where six TeO₅ polyhedra enclose a hexagonal cavity in which a UO₈ polyhedron is located. The [UTe₆O₂₆]^22- clusters are further linked by TeO₅ square pyramids to form the 3D network. Similar to uranyl tellurates, both α-K₂[(UO₂)Te^VIO₅] and β-K₂[(UO₂)Te^VIO₅] contain TeO₆ octahedra which share a common face to form a dimeric Te₂O₁₀ unit. However, in α-K₂[(UO₂)Te^VIO₅], these Te₂O₁₀ units connect with UO₆ tetragonal bipyramids to form a 3D structural framework, while in β-K₂[(UO₂)Te^VIO₅], the same Te₂O₁₀ dimers are observed to link with UO₇ pentagonal bipyramids, forming 2D layers. Raman measurements were carried out and the vibration bands related to Te^IV-O, Te^VI-O and U^VI-O bonds are discussed.

Thorium Chemistry in Oxo-Tellurium System under Extreme Conditions

Through the use of a high-temperature/high-pressure synthesis method, four thorium oxo-tellurium compounds with different tellurium valence states were isolated. The novel inorganic phases illustrate the intrinsic complexity of the actinide tellurium chemistry under extreme conditions of pressure and temperature. Th₂Te₃O₁₁ is the first instance of a mixed-valent oxo-tellurium compound, and at the same time, Te exhibits three different coordination environments (Te^IVO₃, Te^IVO₄, and Te^VIO₆) within a single structure. These three types of Te polyhedra are further fused together, resulting in a [Te₃O₁₁]^8- fragment. Na₄Th₂(Te^VI₃O₁₅) and K₂Th(Te^VIO₄)₃ are the first alkaline thorium tellurates described in the literature. Both compounds are constructed from ThO₉ tricapped trigonal prisms and Te^VIO₆ octahedra. Na₄Th₂(Te^VI₃O₁₅) is a three-dimensional framework based on Th₂O₁₅ and Te₂O₁₀ dimers, while K₂Th(Te^VIO₄)₃ contains tungsten oxide bronze like Te layers linked by ThO₉ polyhedra. The structure of β-Th(Te^IVO₃)(SO₄) is built from infinite thorium chains cross-linked by Te^IVO₃^2- and SO₄^2- anions. Close structural analysis suggests that β-Th(Te^IVO₃)(SO₄) is highly related to the structure of α-Th(SeO₄)₂. Additionally, the Raman spectra are recorded and the characteristic peaks are assigned based on a comparison of reported tellurites or tellurates.